1. Field of the Invention
Embodiments of the invention generally relate to distributed temperature sensing.
2. Description of the Related Art
Distributed Temperature Sensing (DTS) enables monitoring temperature along the length of a well bore, for example. A laser or other light source at the surface of the well transmits a pulse of light into a fiber installed along the length of a well to function as a temperature sensor. As the light propagates through the fiber, scattering reflects some of the light back towards the surface for detection. In Raman scattering, incident light is scattered by optical phonons and undergoes relatively large frequency shifts. In Brillouin scattering, incident light is scattered by acoustic phonons and undergoes relatively small frequency shifts. The frequency or intensity of these reflections, relative to the pulsed light, shift in accordance with the temperature of the atoms along the fiber. Accordingly, processing of this reflected light as a function of time can derive temperature as a function of well depth, with earlier reflections indicating the temperature at shallow depths, and later reflections indicating the temperature at relatively deeper depths. Distributed optical waveguide sensors that use Raman or Brillouin scattering may utilize either Optical Time-Domain Reflectometry (OTDR) or Optical Frequency-Domain Reflectometry (OFDR).
Frequency shifts of Brillouin scattered light depends not only on temperature of the fiber but also on strain conditions of the fiber. Therefore, Brillouin-based DTS generally requires assumptions regarding strain conditions that result in uncertainties in temperature measurements especially since strain may not be constant across the length of the fiber. Further, state of strain on the fiber often changes before and after installation, which complicates even making the assumptions required to discriminate strain from temperature influences.
Raman-based DTS systems rely on light intensity measurements of Raman scattered light to provide temperature determinations, which are not dependent on the strain condition of the fiber under normal circumstances. However, the amplitude of the Raman scattered light is much less than that of the Brillouin scattered light resulting in significantly lower optical loss budgets for Raman-based systems. Problems with Raman DTS may occur as a result of the fiber degrading over time and differential loss. For example, hydrogen causes the fiber to darken when absorbed into the fiber that thus attenuates the light due to this darkening. A further problem with deployment of long lengths of the fiber relates to noise since the noise increases with loss, which increases with length. As a result of high loss, differential loss and fiber degradation, the Raman-based DTS may lack sufficient resolution and sustainable accuracy to continue useful and reliable operation over time across long distances.
Therefore, there exists a need for improved systems and methods of distributed temperature sensing.